Optical microscopy

A method of increasing the signal contrast in interferometric scattering optical microscopy, the method comprising:

• providing a particle detection region having a boundary defined by an interface;
• illuminating a particle in the particle detection region with coherent light using an objective lens such that the light is reflected from the interface and scattered by the particle;
• capturing the reflected light and the scattered light using the objective lens;
• and
• providing the captured reflected and scattered light to an imaging device to image interference between the reflected light and the scattered light;
• wherein the objective lens has an optical axis, the method further comprising
• providing the coherent light to the objective lens offset from the optical axis of the objective lens such that the coherent light illuminating the particle is at an oblique angle to the interface.